This invention relates to the protection of electrical and electronic equipment and, more particularly, to a fusing arrangement for providing current overload protection.
Occasional faults in telephone lines outside of customer premises occur due to lightning phenomenon and accidental coupling of the electrical power distribution plant to the telephone loop plant. In order to safeguard customer telephone station sets, station protectors provide the interface between the outside loop plant and the equipment to be protected. Traditionally, station protectors employ carbon-block, air-gap, or gas-tube devices for over-voltage protection while protection from excessive currents is provided by fusing.
Generally, there are two diverse requirements which should be satisfied for overload current protection in any reliable protection device. First, the ability to reliably interrupt currents in excess of at least 350 amperes at 3,000 volts. Under this condition, the instantaneous vaporization of the fuse wire is likely to produce a potentially destructive power arc due to ionization. The solution to this problem has been to provide arc-extinguishing material surrounding the fuse wire for absorbing the energy from the melting fuse wire to prevent the formation of a power arc. This practice has proved to be successful.
The second requirement is the ability to provide predictable fusing for situations produced by slight over current conditions, which has been difficult due to a number of conditions or properties associated with the fuse wire. The demands on the second requirement have increased due to the introduction of semiconductor devices into telephone station apparatus which are less rugged than the traditional electromechanical components that are being replaced by the semiconductor components. For example, one of the conditions thought to exist on the fuse wire is an oxide buildup on its surface which produces an encapsulating effect for the molten material thereby raising the fusing threshold. Another disturbing effect is that the mounting orientation of the conventional protectors (vertical versus horizontal) affects the fusing characteristic. One approach to this problem has been to restrict the cross-section of the fuse wire with precision to provide a potential hot zone, but this has not resulted in reliable control of a precise fusing point. Furthermore, from a manufacturing standpoint a reliability problem is produced in producing the restricted cross-sectional zone of the fusing wire with the required precision.